1. Technical Field
This invention relates to optical fiber and, in particular, to coated optical fiber.
2. Art Background
Typically, an optical fiber, after it is drawn from a preform, is coated with at least one, and typically two, polymer coatings. These coatings are applied by directing the fiber through a reservoir containing a suitable monomer, drawing the coated fiber through a die, and then curing the monomer into a polymer through exposure to radiation, e.g., ultraviolet radiation. The resulting coatings significantly enhance the mechanical and optical properties of the fiber.
Despite the advantages of polymeric coatings for optical fiber, they are generally permeable to water and hydrogen. This permeation by environmental water or by hydrogen generated during reactions of cable components in applications such as oil well logging or undersea systems has been found to have significant effects. In particular, the interaction of water with the surface of the silica fiber produces surface modifications that lower the fracture resistance of the fiber to applied stress. The interaction of the silica fiber with hydrogen produces an attenuation in the signal carried by the fiber. Thus, the reliability of the optical fiber, especially in adverse environments or the suitability of the fiber for applications where signal attenuation is not acceptable, necessitates a fiber with a hermetic coating alone or in combination with the typical polymer coatings.
Despite this desire, the deposition of hermetic coatings without substantial degradation of properties or substantial increase in cost is extremely difficult to attain. Various attempts have been made to achieve an economic, hermetic coating. For example, in one approach after draw the fiber is directed through a furnace containing a gas. The furnace induces pyrolytic decomposition of the gas which, in turn, produces a coating on the fiber. The composition of the fiber coating depends on the gas employed. Attempts have been made using organic gases to make hermetic coatings that are primarily carbon compositions. In all these attempts the coating was non-adherent and/or non-hermetic. Such attempts have been described in U.S. Pat. No. 4,512,629 issued Apr. 23, 1985, where C.sub.4 H.sub.10 gas was employed; in the SPIE Proceedings on Reliability Considerations in Fiber Optic Applications, Sept. 25-26, 1986, Cambridge, Mass., p. 27, where a C.sub.4 H.sub.10 gas was employed; in the Proceedings of the Optical Fiber Conference, Phoenix, Ariz. 1982, paper WCCl, where the gas utilized was not disclosed; and in Physics of Fiber Optics, Advances in Ceramics, eds. B. Bendow and S. S. Mitra, Vol. 2, pp. 124-133, American Ceramics Society, 1981, where an ion plasma deposition was utilized.
Compositions other than carbon have been produced using the previously described furnace approach for producing hermetic coatings by a gas phase reaction. For example, a combination of C.sub.4 H.sub.10 and TiCl.sub.4 has been utilized to obtain titanium carbide coatings while a combination of silane and ammonia has been utilized to obtain silicon oxynitride coatings. (See SPIE Proceedings on Reliability Considerations in Fiber Optic Applications, Sept. 25-26, 1986, Cambridge, Mass., p. 27, and U.S. Pat. No. 4,512,629, respectively.) The resistance to static fatigue of these coatings, i.e., on the order of n=30-100, for some applications is not entirely acceptable. Additionally, these coatings have typically been applied at draw rates slower than approximately 1 meter per second. Thus, even if the properties of these non-carbonaceous coatings are acceptable, the relatively slow draw speeds compared to typical speeds of 4-6 m/sec substantially increases cost.
Another approach suggested for producing a non-permeable coating employs the heat associated with the fiber after it is drawn to induce decomposition of a gas and subsequent fiber coating. (This approach is mentioned in U.S. Pat. No. 4,575,463 but specifics such as deposition conditions or useful coating precursor gases are not discussed.) Thus, although hermetically coated optical fibers for many applications are desirable, achieving acceptable, economic results is extremely difficult.